Ultra high vacuum isolation valve for volatile materials

ABSTRACT

An ultrahigh vacuum isolation valve comprising a housing having a side tube containing a frangible vial of volatile material and a linear motion feedthrough mechanism in the housing with an enlarged flange portion for rupturing the vial as the valve is opened. A deformable copper seal is held by a retaining screw at a distal end of the linear feedthrough whereat a valve orifice having inclined valve seating surfaces is located.

United States Patent [1 1 Kaldenback et al.

[ 1 Sept. 18, 1973 ULTRA-HIGH VACUUM ISOLATION VALVE FOR VOLATILEMATERIALS [75] Inventors: Kenneth H. Kaldenhack; John Kovals; John H.Pollard, all of Alexandria, Va.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, D.C.

[22] Filed: Oct. 30, 1972 [21] Appl. No.: 302,165

[52] US. Cl 137/68, 137/70, 137/797, 251/335 B, 251/368 [51] Int. Cl.F161: 17/40 [58] Field of Search 137/797, 68, 69, 137/70, 71; 251/335 A,335 B, 368

[56] References Cited UNITED STATES PATENTS 2,937,654 5/1960 Wilner137/68 3,159,377 12/1964 Samour 251/368 X 3,195,552 7/1965 Rasmussen251/368 X 3,202,199 8/1965 Wood 137/71 X 3,209,937 10/1965 Hirst et a1.137/70 X 3,266,505 8/1966 Kron 251/368 X 3,391,901 7/1968 Wheeler et a1.251/368 X 3,512,556 5/1970 McKhann 137/71 3,573,863 4/1971 Doors etal..... 251/335 8 3,590,839 7/1971 Moore 137/71 3,727,406 4/1973Lefeuvre 251/335 B Primary ExaminerSamuel Scott Attorney-Harry M.Saragovitz et al.

[57] ABSTRACT An ultrahigh vacuum isolation valve comprising a housinghaving a side tube containing a frangible vial of volatile material anda linear motion feedthrough mechanism in the housing with an enlargedflange portion for rupturing the vial as the valve is opened. Adeformable copper seal is held by a retaining screw at a distal end ofthe linear feedthrough whereat a valve orifice having inclined valveseating surfaces is located.

1 Claim, 2 Drawing Figures ULTRA-HIGH VACUUM ISOLATION VALVE FORVOLATILE MATERIALS The invention described herein may be manufactured,used, and licensed by or for the Government for governmental purposeswithout the payment to me (or us) of any royalty thereon.

Cesium is the metal most commonly used in photocathode technology toactivate photoemissive materials in order to obtain a low work functionsurface and, thereby achieve efficient photoemission. Only a smallamount of cesium, approximately one monolayer or 10 atoms per squarecentimeter, at the photocathode surface is necessary to do this. Hence,a controllable and reproducible source of cesium flux is of significantpractical interest and importance.

All photocathodes are processed under ultrahigh vacuum conditions and itis desirable that any cesium source operate under the lowest possiblepressure and be controllable while maintaining that pressure. It is alsodesirable that the cesium source be scalable to ultrahigh vacuum thusavoiding the necessity of changing the cesium whenever the main chamberis brought up to ambient pressure for specimen changes.

Known prior art devices suffer from the inability to be scalable toultrahigh vacuum and must be heated to temperatures well over lOC toobtain a useful cesium flux during operation.

The valve described herein incorporates the desirable features of bothan ultrahigh vacuum sealing capability and a useable flux rate at roomtemperature.

SUMMARY OF THE INVENTION The disclosed device is a molecular cesiumsource capable of producing a cesium flux at room temperatures andcapable of repeated use through numerous pressure cycles from ambient toultrahigh vacuum without cesium contamination. The valve sealing isaccomplished using an OFHC copper seal that deforms and engages aconical valve seating surface at the valve orifice.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-sectional view ofthe cesium isolation valve.

FIG. 2 is an enlarged view of the valve closure showing the deformablecopper seal.

DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 1, the devicecomprises an enlarged main body section and a narrower tubular extension11 which extends through the wall 12 of a vacuum chamber 13. The mainbody 10 is joined to a conventional linear motion feedthrough made up ofa screw guide flange 15, drive mechanism 16, thrust bearings 17, drivescrew 18 surrounded by sealing bellows l9 and bearing housing 20. Thelinear motion feedthrough mechanism is sealed to the main body 10 with aconflat seal 21. A knob 26 is fixed to the drive mechanism 16.

A side tube 22, extending from the main body 10 houses a cesium vial 23.The side tube 22 is sealed by a miniconflat seal 24 and cap 25.

vA valve orifice 30, located at the end of the tube 11 has a conicalvalve seating surface 21 inclined at about 45 to the axis of the tube. Ashaft 32 with a flange 33 is bolted to the linear feedthrough screw 18.The closure for the valve orifice 31 shown in FIG. 1, and shown enlargedin FIG. 2, is made up of an OFHC copper seal 34 held on the shaft 32 bya retaining screw and seal 35. As can be seen in FIG. 2, the copper seal34 is drawn back against the conical valve seating surface 31 and isdeformed as shown thereby creating an ultrahigh vacuum seal.

OPERATION OF THE CESIUM ISOLATION VALVE The initial valve seating duringfinal valve assembly is accomplished by torquing the drive mechanism to18 ft lbs; followed by a retightening of the retaining screw 35 into theshaft 32 thereby locking the copper gasket in place. Subsequent sealingto ultra-high vacuum only requires 15 ft lbs of torque on the drivemechanism. The valve is normally heliarc welded on a rotatable flange (aportion of which is shown at 12) and is positioned to give line of sightfrom the sample surface 36, being activated, to the interior of thevalve through the valve orifice 30. The cesium vial 23 is introducedinto the side tube 22 and sealed with the miniconflat seal 24 and cap25; with the valve orifice 30 in a partially open position. The completevalve and vacuum system is pumped down and baked for 24 hours at 200Cand allowed to cool to attain ultrahigh vacuum conditions of 10 torrpressure or lower. The valve is now fully opened breaking the vial 23and releasing cesium into the main body 10 of the valve. With the valvethen closed, cesium is vacuum distilled uniformly upon the interiorsurfaces of the valve by gently heating the external portions of thevalve with a heating tape. Upon subsequent cooling the valve is readyfor use.

RESULTS OF VALVE USE l. The valve seals consistently to 10* torr orbetter. One particular valve has operated satisfactorily for a full yearwithout replenishment of the cesium in a research system.

1 2. Opening and closing the valve does not increase the system pressureby greater than 2 X 10' torr during use; resulting in a cesium sourcefree of gaseous contamination.

3. Line of sight mass spectrometry tests showed the only detectableimpurity to be 25 parts per million of rubidium.

4. The mass spectrometer has been used to calibrate the source fluxrate. The rate with the valve at room temperature was l0 atoms/cm /secat a sample surface 4 centimeters from the valve axis at the valveorifice. This value is in agreement with calculations of the flux fromsimple kinetic theory using published values for the vapor pressure ofcesium metal.

5. Auger spectroscopy measurements of the cesiated surface show the fluxrate to be consistently reproducible.

ADVANTAGES OF THE VALVE 1. The valve provides ultrahigh vacuum sealingallowing the vacuum system to be brought up to ambient pressure withoutnecessitating a change of cesium.

2. The cesium source is highly pure and needs no outgassing.

3. The source operates at room temperature providing a monolayercoverage of the sample in ten minutes.

4. Outgassing problems associated with high temperature operation areavoided completely.

5. The source need only be charged with cesium once.

6. The cesium flux rate can be accurately and practicallyinstantaneously controlled.

7. Experimental tests show the flux rate is completely reproducible.

While a preferred embodiment of the invention has been described, itshould be understood that variations thereof, substitutions andalterations, such as the use of tending therein;

a sealable side tube extending laterally from said main body for housinga sealed frangible vial of volatile material and so positioned that thehead of said drive screw will break the vial as it advances to an openvalve position;

a laterally placed valve orifice at the distal end of the tubularextension, said orifice having a conical valve seating surface;

a shaft affixed to the head of said drive screw and carried in thetubular extension;

valve orifice sealing means including a deformable copper disc on theend of said shaft, said copper disc being retained on said shaft by aretaining screw and seal member and being engageable with said conicalvalve seating surface for sealing said valve to ultrahigh vacuum.

1. An ultrahigh vacuum isolation valve comprising: a housing having anenlarged main body and an elongated narrow tubular extension thereon; alinear motion feedthrough sealedly affixed to the enlarged main body andhaving a drive screw extending therein; a sealable side tube extendinglaterally from said main body for housing a sealed frangible vial ofvolatile material and so positioned that the head of said drive screwwill break the vial as it advances to an open valve position; alaterally placed valve orifice at the distal end of the tubularextension, said orifice having a conical valve seating surface; a shaftaffixed to the head of said drive screw and carried in the tubularextension; valve orifice sealing means including a deformable copperdisc on the end of said shaft, said copper disc being retained on saidshaft by a retaining screw and seal member and being engageable withsaid conical valve seating surface for sealing said valve to ultrahighvacuum.